Radio Frequency Identification (RF ID) is just one of many identification technologies for identifying objects. The heart of the RF ID system lies in an information carrying tag. The tag functions in response to a coded RF signal received from a base station. Typically, the tag reflects the incident RF carrier back to the base station. Information is transferred as the reflected signal is modulated by the tag according to its programmed information protocol.
The tag consists of a semiconductor chip having RF circuits, logic, and memory. The tag also has an antenna, often a collection of discrete components, capacitors and diodes, for example, a battery in the case of active tags, a substrate for mounting the components, interconnections between components, and a means of physical enclosure. One variety of tag, passive tags, has no battery. They derive their energy from the RF signal used to interrogate the tag. In general, RF ID tags are manufactured by mounting the individual elements to a circuit card. This is done by using either short wire bond connections or soldered connections between the board and the circuit elements: chip, capacitors, diodes, antenna. The circuit card may be of epoxy-fiberglass composition or ceramic. The antennas are generally loops of wire soldered to the circuit card or consist of metal etched or plated on a circuit card. The whole assembly may be enclosed in a plastic box or molded into a three dimensional plastic package.
While the application of RF ID technology is not as widespread as other ID technologies, bar code for example, RF ID is on its way to becoming a pervasive technology in some areas, notably vehicle identification.
Growth in RF ID has been inhibited by the high cost of tags, the bulkiness of most of the tags, and problems of tag sensitivity and range. A typical tag costs in the $5 to $10 range.
Companies have focused on niche applications. Some prior art is used to identify railway boxcars. These tags tend to be quite large and are made of discrete components on circuit boards mounted in solid, non-flexible casings. RF tags are now used in the automatic toll industry, e.g. on thruway and bridge tolls. RF tags are being tested for uses as contactless fare cards for buses. Employee identification badges and security badges have been produced. Animal identification tags are also commercially available as are RF ID systems for tracking components in manufacturing processes.
Tags exist that have the-length and width of a standard credit card. However, these cards typically are over 2.5 mm thick and have a non-flexible casing. Tags also exist that have a credit card size length and width but with bumps where circuit is placed that causes them to be too thick to fit in card reader machinery
While some electronic article surveillance (EAS), e.g. antitheft devices, are thin (0.3 mm) they typically contain limited amounts, (i.e., only one bit) of information. Some of these devices can be turned off once but cannot be reactivated.
FIG. 1A shows one prior art structure of a radio frequency tag 105. The tag 105 has a chip 110 mounted on a substrate 115. The chip 110 has contacts 120 that are connected to circuitry on the substrate 115 by wire bonds 125. An encapsulation material 130 covers the chip for environmental protection. The thickness of this tag 105 is determined by the combined thicknesses of the chip components. Typically, substrates in these tags are at least 10 mils, 0.25 mm, in thickness, the chip 110 along with the high loop 122 of the bond vary from 20 to 40 mils, 0.5 to 1 mm, in thickness and the encapsulation 130 is about 10 mils, 0.25 mm in thickness. As a result, tags 105 of this structure vary from a minimum of 40 to 60 mils, 1 to 1.5 mm, in thickness. This structure is too thick for many potential tag applications.
FIG. 1B shows another prior art structure 150 showing a chip 110 with the chip contacts 120 connected to circuitry contacts 155 with conducting adhesive 160. The substrate 165 of this structure 150 is typically made as a FR4/printed circuit (thickness 40 to 60 mils, 1 to 1.5 mm) or flexible substrate (10 mils,0.25 mm). The chip 110 and adhesive 160 add another 20 to 40 mils, 0.5 to 1 mm, to the thickness and the encapsulation 130 adds still another 10 to 20, 0.25 to 0.5 mm mils in structure 150 thickness. This structure therefore can vary in thickness from 80 to 130 mils, 2 to 3.5 mm, making it thicker than the structure in FIG. 1A.
Other thick structures are known in the art. These include quad flat pak (QFP) and/or small outline pak (SOP) as components. Structures made with these components are at least 1 mm thick and usually 2 to 3 mm thick.